Electro-plating of metals



Uni d. S ms Pa n 3,003,933 ELECTRO-PLATING 0F METALS James Thomas Nesbitt Atkinson, Dartmouth, Nova Seotia, Canada, assignor to Her Majesty the Queen in the right of Canada as represented by the Minister of National Defence N0 Drawing. Filed Apr. 21, 1958, Ser. No. 731,073 3 Claims. (Cl. 204-49) This invention relates to a new plating process whereby satisfactory nickel plates may be deposited upon a number of metals including particularly aluminum and aluminum base alloys which hitherto have been considered refractory to normal plating methods. The present application is a continuation-in-part of my application Serial No. 544,600, filed November 2, 1955, now Patent No. 2,871,172, which was in turn a continuation-in-part of my application Serial No. 468,533, filed November 12, 1954, and now abandoned.

The present invention will be described with particular reference to the direct plating of nickel on aluminum and aluminum base alloys in as much as the plating of aluminum presents particular difficulties and nickel plated aluminum has Wide applications, particularly in the home appliance field. Nevertheless, the present invention is also applicable to the plating of a large number of other metals such as zinc, iron, steel, copper, brass and the like.

Previously known methods of plating nickel on aluminum have been very complicated and involve the use of a series of mechanical or chemical pretreatments by means of which the natural protective oxide coating is removed or replaced for instance with a zinc coating or a modified oxide film. While fairly satisfactory plates can be achieved, these processes are expensive and the achievement of uniform results requires a high degree of precision in control of the various steps and the racking of the articles which are to be plated. Commercial quantity production on an economic basis such as can be achieved by the well known barrel-plating process or by continuous wire or strip plating are generally unprofitable or impractical.

I have found that certain strongly acid solutions have a pH of about 4 or less and having properties of pickling solutions as described more particularly below, are especially useful for the direct plating of nickel on aluminum.

The essential components of the plating solution of my invention are a suitable form of nickel, the sulphate being preferred on account of its ready availability, in solution along with a fiuoborate or fluosilicate. In certain circumstances other ingredients may be highly desirable such as halides of atomic weight between 35 and 127, and boric acid. Chlorides are the most useful halide to employ; replacement of chlorides by bromides or iodides is presently undesirable economically, although electrochemically feasible. Boric acid is a desirable but not normally essential additive; its principal function is believed to be that of promoting maintenance of a stable operating pH of the plating solution, butwhatever its exact function it is in any event a common constituent of most nickel plating solutions. The use of fiuosilicates rather than fluoborates may be. useful in certain circumstances but has the great disadvantage of greatly shortening the useful life of the plating solution owing to gel formation.

The pickling solution as referred to above, is an acid solution with a high dissolving power for the oxide of the cathodic metal concerned, yet with no appreciable dissolving power for the cathodic metal itself. Where the cathodic metal forms a stable oxide film, as in the case of aluminum and aluminum base alloys for instance, a

2 closely controlled additionof the halide constituent of the plating solution leads to optimum pickling properties.

In addition to a specific effect on stable oxide films, halides promote smooth anode corrosion and hence for this reason are generally a desirable constituent of the solution. Where however the cathodic material is alu-' minum or an aluminum base alloy addition of halide must be limited to a concentration not greater than about 0.01 N otherwise undue attack upon the cathodic material is encountered.

Appropriate acids to be used for adjusting the pH into the desired range will be apparent to those skilled in the art. Generally, since the sulphate and fluoborate or fluosilicate acid radicals are already present in the plating solution in appreciable amounts, sulphuric acid or fiuoboric or fiuosilicic acid is preferred. Other non-interfering acids may be employed but clearly hydrohalic;

acids cannot be employed to any great extent where the total concentration of halide is to be limited. Should it be desired to adjust the pH in an upward direction ammonium hydroxide or other non-interfering alkali may be employed. I

The plating solutions may be further modified by the addition of brightening agents, wetting agents, buffers and other additives. It is, of course, not possible to list exhaustively all additions that could be made for improving the various properties of such a plating solution; those skilled in the art will, however, readily recognize the additions likely to have a further beneficial effect.

Suitable proportions of the various ingredients of the plating solution will be apparent to those skilled in the art. For instance good results have been achieved using 100 to 400 grams of nickel sulphate crystals, 300 to millilitres of nickel fiuoborate solution (specific gravity 1.49), A to 1 gram of nickel chloride crystals, 10 to 40 grams boric acid, dissolved in sufficient water to make 1 litre. The concentration of nickel salts in the bath are inter-dependent because of the common ion solubility effect. Combinations exceeding the solubility of nickel should not be used. Such a solution will be somewhat acid in any event and may be adjusted to a pH of about 0.9 to 1.5, and may be used at temperatures of about 50 to 70 C. with a nickel anode and a cathode current density of from about 30 amperes per square foot for striking to about 750 amperes per square foot or more for electro-forming. Plating at excessively high plating bath is important as in many other nickel plating processes known in the art, and is particularly useful for achieving satisfactory plating at higher current densities.-

The process of the invention requires a simple cleaning operation only prior to plating. This is given in detail for aluminum; similarly simple techniques, ob-

vious to those skilled in the art, may be used for other materials. If the aluminum'is heavily soiled an initial vapour degreasing ,step maybe desirable, but a mild alkaline cleaner should be used in anyfcase. For example, irnmersion cleaning in an aqueous solution containing 3% each of sodium carbonate and trisodium phosphate for about two minutes at 60 C. or a shorter period of cathodic cleaning is suitable for pure aluminum or the more corrosion resistant alloys. Some care should be taken not to leave the work in the alkaline solution for too long a time, particularly with the less corrosion resistant alloys; these latter tolerate shorter cleaning times only. The best general rule is to continue the cleaning until the appearance resembles that of a sample of 28 alloy cleaned for a two-minute periodin afreshly prepared cleaner as given above. Work which has been inadvertently overcleaned may be treated with a suitable acid to remove smut, following which it should be reprocessed in the alkaline cleaner, and rinsed 'as usual.

In addition to the ability of my plating solutions to plate directly on a variety of cathode materials, the solution may be used for relatively thick deposits of nickel at a high rate of deposition limited essentially by the agitation of the bath and the current distribution in the work to be plated rather than by any failing of the solution itself. While the emphasis in the foregoing description has been on the plating of aluminum and aluminum base alloys, the general use of the solution resembles that of normal nickel plating solutions except that stainless steel may be plated directly without activation procedures, and that this surprisingly acid solution is also suitable for plating on zinc. In these properties the plating solution furnishes a new and useful combination of desirable qualities.

The adhesion of my direct plating on aluminum commonly exceeds the ultimate strength of tin-lead solder in shear. Adhesion may be further improved by heating the plated work for about 10 minutes to 500 C., giving adhesion values of at least 10,000 psi in shear consistently.

The process of the invention is illustrated by the following examples which it will be appreciated are exemplary only and are not intended in any sense to indicate the scope of the invention.

Example 1 A suitable composition for nickel plating as disclosed above consists of:

Nickel sulphate crystals ..g. 100 400 Nickel fluoborate solution (Sp. g. 1.49) "ml" 306 75 Nickel chloride crystals 1 Borlc acid 1O 40 pH adjusted to"-.. 0.9 to 1.5 Water to make 1 litre Example 2 A preferred composition for such nickel plating is: Nickel sulphate crystals g 200 Nickel fiuoborate (sp. g. 1.49) ml 150 Nickel chloride crystals g 1 Boric acid g 20 pH adjusted to 0.9-15. Water to make 1 litre.

Example 3 As Example 2, with nickel fluosilicate solution replacing nickel fluoborate solution.

This composition gives good plating until the solution gels, after which it becomes useless.

Example 4 As Example 2, with nickel chloride replaced by a chemically equivalent amount of another suitable chloride such as cobalt chloride or ammonium chloride.

Example 5 As Example 2, with nickel chloride replaced by a chemd ically equivalent amount of a suitable halide such as potassium bromide or potassium iodide.

This bath should be operated at a temperature 50-60 C. and a cathodic current density of 50-150 amperes per square foot.

Example 8 A plating bath was made up as follows:

Boric acid g 40 Nickel sulphate crystals 150 Nickel fluosil 50 Water to make 1 litre. Final pH was 2.7.

A current is passed through this solution at room temperature using a clean piece of aluminum or aluminum alloy as cathode, the current density being 0.12 ampere per square inch. The resulting nickel plate is bright and of good appearance. Adhesion, measured after the sample has stood for some hours, is satisfactory. Similar results are obtained when the experiment is repeated with pI-Is ranging up to about 4.0, the pH being adjusted upwardly with ammonia.

While useful results can be obtained at pI-Is up to about 4.0, the most versatile and generally useful baths are those which are operated below about pH 2.0.

What I claim is:

1. A method of electro-plating nickel directly onto the surface of a clean cathodic metal member selected from the group consisting of aluminum and aluminum base alloys which comprises: immersing an anode and said cathodic metal member into a plating bath solution having a pH not over about 4.0, said solution containing dissolved therein per litre about to 400 grams of crystalline nickel sulphate, about 300 to 75 millilitres of nickel fluoborate solution (specific gravity 1.49), from about /4 to 1 gram of crystalline nickel chloride, and from about 10 to 40 grams of boric acid, and passing an electric current through said solution between said cathodic metal member and said anode until a nickel plate of the desired thickness has been deposited on said cathodic metal member.

2. A method of electro-plating nickel directly onto the surface of a clean cathodic metal member selected from the group consisting of aluminum and aluminum base alloys which comprises: immersing an anode and said cathodic metal member into a plating bath solution having a pH between about 0.9 and 1.5, said solution containing dissolved therein per litre about 100 to 400 grams of crystalline nickel sulphate, about 300 to 75 millilitres of nickel fluoborate solution (specific gravity 1.49), from about /4 to 1 gram of crystalline nickel chloride, and from about 10 to 40 grams of boric acid, and passing an electric current through said solution between said cathodic metal member and said anode until a nickel plate of the desired thickness has been deposited on said cathodic metal member.

3. A method of electro-plating nickel directly onto the surface of a clean cathodic metal member selected from the group consisting of aluminum and aluminum base alloys which comprises: immersing an anode and said cathodic metal member into a plating bath solution having a pH between about 0.9 and 1.5, said solution contained dissolved therein per litre about 200 grams of crystalline nickel sulphate, about 150 millilitres of nickel fluoborate solution (specific gravity 1.49), about 1 gram of crystalline nickel chloride and about 20 grams of boric acid, and passing an electric current through said solution between said cathodic metal member and said anode until a nickel plate of the desired thickness has been deposited on said cathodic metal member.

References Cited in the file of this patent UNITED STATES PATENTS Kern v Dec. 7, 1909 Frasch Mar. 4, 1941 Blackmun et a1 Feb. 23, 1943 Brown Aug. 4, 1953 Atkinson Jan. 27, 1959 

1. A METHOD OF ELECTRO-PLATING NICKEL DIRECTLY ONTO THE SURFACE OF A CLEAN CATHODIC METAL MEMBER SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM BASE ALLOYS WHICH COMPRISES: IMMERSING AN ANODE AND SAID CATHODIC METAL MEMBER INTO A PLATING BATH SOLUTION HAVING A PH NOT OVER ABOUT 4.0, SAID SOLUTION CONTAINING DISSOLVED THEREIN PER LITRE ABOUT 100 TO 400 GRAMS OF CRYSTALLINE NICKEL SULPHATE, ABOUT 300 TO 75 MILLILITRES OF NICKEL FLUOBORATE SOLUTION (SPECIFIC GRAVITY 1.49), FROM ABOUT 1/4 TO 1 GRAM OF CRYSTALLINE NICKEL CHLORIDE, AND FROM ABOUT 10 TO 40 GRAMS OF BORIC ACID, AND PASSING AN ELECTRIC CURRENT THROUGH SAID SOLUTION BETWEEN SAID CATHODIC METAL MEMBER AND SAID ANODE UNTIL A NICKEL PLATE OF THE DESIRED THICKNESS HAS BEEN DEPOSITED ON SAID CATHODIC METAL MEMBER. 